Air is Free, but Not Carefree

We concluded that air gaging represents the method of choice for most high-resolution measurements on large production runs. While quite durable and reliable compared to mechanical gages, air gaging is not care-free. Accurate air gaging requires proper maintenance of the tooling, and vigilance over the air supply. Although the factory air supply may not be under the gage user’s control-- compressors and air lines may be shared by dozens of other users--the gage user must ensure that the air reaching his gage is clean, dry, and fairly stable in pressure. Tooling, on the other hand, is directly under the gage user’s control, and he is responsible for its maintenance.

Proper tool maintenance simply means keeping it clean and dry inside and out. Contaminants such as chips, dirt, coolant, and cutting fluid may be picked up from workpieces, while water and oil are likely to come from the air source itself. Although the air flow tends to clear out most air passages on its own, some contamination may occur in the master jet or measuring jets. Even accumulations of only a few millionths of an inch can throw off a measurement. The gage must be inspected and cleaned when necessary. Repeated mastering that produces varying readings is a good indication of dirty jets. Shop air is difficult to keep clean and dry. Air dryers are not entirely adequate. The very act of compressing air produces moisture, and a compressor’s need for lubrication inevitably generates some oil mist in the line. Oil and water mist can actually act as an abrasive and cause part wear over long periods (for example, the Grand Canyon), so don’t leave the gage on overnight. Our near-term goal, however, is simply to prevent mist from entering the gage and fouling the jets. To do this, we must employ proper air-line design to intercept it before it enters the meter. Air main lines should be pitched down from the source, with a proper trap installed on the end. Feed lines should also be equipped with traps. Take air from the top rather than Section K 5 the bottom of the mains, so that moisture doesn’t drain into the fee. Long, gentle bends on feeds are preferable to hard angles and close ells. Bleed air lines before connecting gages to them. Gages must always have a filter in place when operating, and this should be changed when it becomes saturated. So, enough about moisture and that oil.

Let’s talk about air. Air leaks are another common cause of air gage inaccuracy. To test, cover the measuring jets tightly with your fingers and observe the indicator needle. If it’s not stationary, check all fittings, tubes and connectors for leaks. Simple. Most factory air lines run at about 100 psi, but depending upon the demands of other air users, this can fluctuate widely. Properly designed air gages operate reliably over a wide range--some as wide as 40-150 psi--so a certain amount of fluctuation is acceptable. Other gages are more sensitive and must be isolated from fluctuations by using a dedicated or semidedicated air line. To check the sensitivity, simply leave a master in place on the tool and observe the indicator for movement as other air-line users perform their normal tasks.

If large pneumatic equipment is being used on the same air line, surges over 400 psi might be generated that could blow out the built-in regulator and damage the gage itself. Again, isolation of the line is the solution. Tight, clean and dry: the requirements of air gaging aren’t very different from mechanical gaging after all. On some highly polished or lapped workpieces, mechanical gage contacts can leave visible marks. Air gaging, as a non-contact operation, won’t mark fine surfaces. For the same reason, air gaging can be more appropriate for use on workpieces that are extremely thin-walled, made of soft materials, or otherwise delicate. Continuous processes, as in the production of any kind of sheet stock, rolled or extruded shapes, also benefit from non-contact gaging.

Air equipment can save time in almost any gaging task that is not entirely straightforward. Air plugs with separate circuits can take several measurements simultaneously on a Section K 6 single workpiece, for example, to measure diameters at the top and bottom of a bore for absolute dimensions, or to check for taper. Jets can be placed very close together for measurements of closely spaced features. Air plugs are available (or can be readily engineered as “specials”) to measure a wide range of shapes that would be difficult with mechanical tools. Examples include: spherical surfaces, interrupted bores, tapered bores, and slots with rectangular or other profile shapes.

t would be possible to design a fixture gage with a number of dial indicators to measure several dimensions in a single setup, such as diameters of all the bearing journals on a crankshaft. But a fixture gage using air gaging will almost inevitably be simpler in design and fabrication, easier to use, less expensive and more accurate. Because of the relative simplicity of fixture design, air gaging is especially suited to relational, as opposed to dimensional, measurements, such as squareness (see illustration), taper, twist, parallelism, and concentricity.

Air gaging isn’t perfect, though. Its high level of resolution makes air gaging impractical for use on workpieces with surface finish rougher than 50 microinches Ra because the readings would average the highs and lows of the rough surface. Most important, air gaging has relatively high initial cost, so it is usually reserved for large production runs. Clean, compressed air is also expensive to generate and must be figured into the equation. In general, however, air gaging is the fast, economic choice for measuring large production runs and/or tight tolerances.

Written by George Schuetz, Director of Precision Gages, Mahr Federal Inc.


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